This invention relates to steam generation systems of the type which includes a selective catalytic reduction capability for catalytically treating flue gas created by the combustion process and, more particularly, relates to a damper vane assembly for such a system operable to regulate the flow of flue gas to a selective catalytic reduction apparatus (SCR).
The reduction of nitrogen oxides (NO.sub.x) emissions from stationary combustion sources, such as fossil fuel-fired power generation systems, has become a critical issue in many nations. As a result, the technology associated with the control of NO.sub.x emissions from fossil fuel-fired power generation systems has matured and expanded significantly. The NO.sub.x emissions reduction processes available for use with fossil fuel-fired power generation systems through NO.sub.x control within the fossil fuel-fired steam generator, such as by means of, for example, overfire air, gas recirculation, reduced-excess-air firing, gas mixing, low-NO.sub.x concentric tangential firing, staged combustion, fluidized-bed firing, etc.; and through post combustion NO.sub.x control effected downstream of the fossil fuel-fired steam generator primarily through the use of selective catalytic reduction (SCR) equipment, provide several alternatives for meeting strict nitrogen-oxide emission levels. Depending on the NO.sub.x emission level required, an optimum NO.sub.x reduction system may result in the integration of several of the above techniques in the overall fossil fuel-fired power generation system.
An example of a fossil fuel-fired power generation systems having NO.sub.x reduction equipment incorporated therewithin is disclosed in U.S. Pat. No. 4,220,633, which issued on Sep. 2, 1980 and which is entitled "Filter House and Method for Simultaneously Removing NO.sub.x and Particulate Matter from a Gas Stream." In accordance with the teachings of U.S. Pat. No. 4,220,633, there is provided a vapor generator and a filter house, the latter being disposed between the vapor generator and an air preheater. An ammonia storage tank is positioned to introduce ammonia via an ammonia distribution grid into the flue gas inlet conduit through which flue gas is transported from the vapor generator to the filter house. The filter house is designed to be operative for removing or cleansing NO.sub.x emissions from the flue gas stream transported thereto during the passage thereof through the filter house while simultaneously filtering out entrained particulate matter from the same flue gas stream. The selective catalytic reduction process was originally developed for those applications where strict NO.sub.x emission requirements dictate the use of post-combustion NO.sub.x reduction techniques. The selective catalytic reduction process was initially applied to natural gas-fired power generation systems, then to low and high sulfur oil-fired power generation systems, and finally to coal-fired power generation systems.
The selective catalytic reduction system uses a catalyst and a reductant, e.g., ammonia gas, i.e., NH.sub.3, to dissociate NO.sub.x to nitrogen gas and water vapor. The catalytic-reactor chamber is typically located between the economizer outlet of the fossil fuel-fired steam generator and the flue-gas inlet of the air preheater of the fossil fuel-fired power generation system. This location is typical for fossil fuel-fired power generation systems with selective catalytic reduction system operating temperatures of 575.degree. F. to 750.degree. F., i.e., 300.degree. C. to 400.degree. C.
Upstream of the selective catalytic reduction chamber are the ammonia injection pipes, nozzles, and mixing grid. Through orifice openings in the ammonia injection nozzles, a diluted mixture of ammonia gas in air is dispersed into the flue-gas stream. After the mixture diffuses, it is further distributed in the gas stream by a grid of carbon steel piping in the flue-gas duct. The ammonia/flue-gas mixture then enters the reactor where the catalytic reaction is completed.
The flue gas which is treated in a selective catalytic reduction arrangement such as an SCR is typically flowed to the SCR via dedicated duct work which is branched from other duct work in the back pass region of the fossil fuel fired power generation system. It can be advantageous to provide a flue gas flow arrangement which permits regulating the flow of flue gas such that the SCR can be isolated from the flue gas during certain times such as, for example, when it is desired to place the SCR out of service or to perform maintenance on the SCR. It is known to provide a combination of dampers operable within the flue gas duct work to effect such an isolation of the SCR. Dampers for selective diversion of flue gas are known such as, for example, the damper disclosed in U.S. Pat. No. 3,897,773 which issued on Aug. 5, 1975 and is entitled "Damper". This reference discloses a blade damper operable to isolate a heat exchanger so that flue gases do not flow thereto but, instead, flow to a bypass stack.
Moreover, insofar as the optimization of the operating efficiency of a fossil fuel-fired power generation system that incorporates selective catalytic reduction is concerned, such optimization of the operating efficiency thereof must be attained while yet ensuring that the operating temperature requirements of the SCR are satisfied. In this regard, as has been noted herein previously, in order to realize the performance desired therefrom the SCR must be located within the fossil fuel-fired power generation system such that the operating temperature to which the SCR is subjected is between 575.degree. F. and 750.degree. F. Not only must this temperature range be maintained for the SCR when the fossil fuel-fired power generation is being operated at full load, but also must be maintained when the fossil fuel-fired power generation system is being operated other than at full load--in other words, operation at a partial load. In order to maximize the flexibility of controlling the gas temperature leaving the fossil fuel-fired power generation system, a flue gas bypass around a portion of the heat transfer surface thereof can be utilized. Thus, in the event of an operation at partial load, it may be desirable to regulate the flow of flue gas to the SCR through such a bypass duct and such routing of the flue gas through the bypass duct can be aided by an appropriate damper arrangement.
In the case of fossil fuel-fired power generation systems that incorporate an SCR therewithin, there is a need for a capability to regulate the flow of flue gas relative to the SCR to permit isolation of the SCR from flue gas during certain times such as out of service times and maintenance times. Also, there is a need in connection with the optimization of the operating efficiency of such a fossil fuel-fired power generation system to ensure adequate volumes of flue gas can be flowed to the SCR via a bypass duct during operation at a low load.